%0 Journal Article %J Physical Review B %D 1978 %T Far-infrared frequency-dependence of the AC Josephson effect in niobium point contacts %A Weitz, D. A. %A Skocpol, W. J. %A Tinkham, M. %X We have measured the far-infrared frequency dependence of the strength of the ac Josephson effect in Nb cat-whisker point contacts, which have consistent and reproducible behavior and minimal extrinsic high-frequency limitations due to capacitance and heating. We monitor the constant-voltage Josephson steps induced on the dc I−V curves by an optically pumped far-infrared laser at fundamental frequencies corresponding to voltages from ∼0.2 to ∼2 times the energy-gap voltage. At all the frequencies studied, we find that the shape of the power dependence of the step amplitudes is fit reasonably well by Werthamer's frequency-dependent theory for tunnel junctions in the voltage-bias approximation. However, the observed magnitude of the steps is considerably less than predicted by the theory. By fitting to the I−V curves of the steps, we find that some of this discrepancy can be accounted for by heating-enhanced noise rounding. The remaining discrepancies (of the order of a factor of 2) are attributed to departures from a voltage bias at low frequencies and, tentatively, to the effects of the Ginzburg-Landau relaxation time at higher voltages. Our data confirm the expected intrinsic roll off of the strength of the ac Josephson effect above the energy gap. %B Physical Review B %V 18 %P 3282-3292 %G eng %U https://journals.aps.org/prb/abstract/10.1103/PhysRevB.18.3282 %N 7